1. Field of the Invention
The present invention relates to an optical fiber grating technology, particularly to a method for modulating refractive indices of optical fiber gratings.
2. Description of the Related Art
FBG (Fiber Bragg Grating) is a key element for fiber communication. FBG functions as a filter to reflect the incident light with a wavelength meeting the Bragg condition and permit the light having other wavelengths to pass. FBG is extensively used in WDM (Wavelength Division Multiplexing) systems, DWDM (Dense Wavelength Division Multiplexing) systems, fiber sensor technology and fiber laser technology.
In FBG fabrication, the cost and quality correlates closely with the variation of the refractive index of the fiber core. Recently, many FBG fabrication technologies have been proposed. A Taiwan patent pending of application No. 200515020 utilizes two polarized light beams, which are vertical to each other, to fabricate FBG, wherein one beam is used to write the fiber and modulate the refractive index, and the other beam is used to maintain the total exposure intensity at a given value. As this method has to control polarization, it needs additional optical elements to work, such as lenses, half-wave plates, and polarizing beam splitters, which will raise the cost. A Taiwan patent pending of application No. 200515021 controls the polarization direction of one light beam in a two beam interference method to maintain the total intensity at a given value, wherein the intensities of the interference fringes are modulated via the relative polarization directions of two light beams. This method can only apply to the two beam interference method. As this method also has to control polarization, it also needs additional optical elements, and the cost thereof also increases. In a paper, by J. B. Jensen, et al., in Optics Letters 2002, p. 1004, the polarization directions are controlled in the phase mask method to maintain the total exposure at a given intensity, and the intensities of the interference fringes are modulated with the intensities of two light beams having different polarization directions. This method can only apply to the phase mask method. As this method also has to control polarization, it also needs additional optical elements, such as half-wave plates and polarizers, and the cost thereof also increases.
In a U.S. Pat. No. 5,830,622, some specified positions are exposed to UV (Ultra-Violet) light to adjust the refractive index thereof and introduce additional phase shifts. However, this method needs double UV exposures, which is time-consuming. Further, it is hard to obtain the desired phase shifts section by section. In a paper, by J. Albert, et al., in Electronics Letters, 1995, p. 223, an optical fiber is written with a special phase mask. However, the length of the fiber grating will be limited by the length of the phase mask. Further, the special phase mask increases the cost. Besides, the method lacks the flexibility to fabricate other specifications of fiber gratings but can only fabricate a special specification fiber grating. In a paper, by M. J. Cole, et al., in Electronics Letters, 1995, p. 1488, a fuzzy technology is used to modulate the refractive index. In this method, a perturbation error is likely to be introduced into the length of the fiber grating. Further, the dc index does not maintain constant but has a slight perturbation.
Accordingly, the present invention proposes a novel method for modulating the refractive indices of optical fiber gratings to overcome the abovementioned problems.